Corrosion-resistant aluminum alloy

ABSTRACT

An extrudable, drawable and brazeable aluminum alloy that has improved corrosion resistance and is suitable for use in thin-wall fluid-carrying tube lines. Preferred alloys consist essentially of, by weight, about 0.17 to about 0.22% iron, about 0.06 to about 0.10% silicon, about 0.30 to about 0.70% manganese, about 0.10 to about 0.30% magnesium, and about 0.19 to about 0.25% zinc, with the balance aluminum and incidental impurities.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This is a continuation-in-part patent application of co-pending U.S. patent application Ser. No. 09/291,255, filed Apr. 13, 1999, now abandoned, which claimed the benefit and priority of European patent application No. 99200493.7, filed Feb. 22, 1999.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

[0002] Not applicable.

BACKGROUND OF THE INVENTION

[0003] (1) Field of the Invention

[0004] The invention relates to a corrosion-resistant aluminum alloy, especially an alloy intended to be used for manufacture of automotive air-conditioning tubes for applications as heat exchanger tubing or refrigerant-carrying tube lines, or generally fluid-carrying tube lines. The alloy has extensively improved resistance to pitting corrosion and enhanced properties in bending and end-forming.

[0005] (2) Description of the Related Art

[0006] The introduction of aluminum alloy materials for automotive heat exchange components is now widespread, applications including both engine cooling and air-conditioning systems. In air-conditioning systems, aluminum components include the condenser, the evaporator and the refrigerant routing lines or fluid-carrying lines. In service these components may be subjected to conditions that include mechanical loading, vibration, stone impingement and road chemicals (e.g., salt water environments during winter driving conditions). Wrought aluminum alloys of the M3000 series type have found extensive use for these applications due to their combination of relatively high strength, light weight, corrosion resistance and extrudability.

[0007] As an example of the above, the abstract of Japanese Patent 05263173 discloses aluminum alloys described as being suitable for heat exchanger fins. The abstract does not provide any indication of the properties except that they are brazeable. The ranges stated in the abstract for the constituents of the alloys broadly encompass a significant number of the above-noted AA3000 series aluminum alloys, including the well-known AA3102, 3103 and 3003 alloys. What particular proportions of the constituents yield an alloy suitable for a heat exchanger fin, and whether such alloys are suitable for any other purpose, are not apparent from the abstract of 05263173.

[0008] To meet rising consumer expectations for durability, automobile producers have targeted a ten-year service life for engine coolant and air-conditioning heat exchanger systems. The AA3000 series alloys (such as M3102, AA3003 and AA3103), however, suffer from extensive pitting corrosion when subjected to corrosive environments, leading to failure of the automotive component. Therefore, to be able to meet the rising targets/requirements for longer life in automotive systems, new alloys have been developed with significantly better corrosion resistance. Especially for condenser tubing, ‘long life’ alloy alternatives have recently been developed, such as those disclosed in U.S. Pat. No. 5,286,316 and WO97/46726. The alloys disclosed in these patents are generally alternatives to the standard AA3102 or AA1100 alloys used for condenser tube, i.e., extruded tube material of relatively low mechanical strength. Due to the improved corrosion performance of the condenser tubing, the corrosion focus has shifted towards the next area to fail, the manifold and the refrigerant-carrying tube lines. Additionally, the tendency towards using more underbody/vehicle tube runs, e.g., rear climate control systems, requires improved alloys due to the greater exposure to the road environment.

[0009] The fluid-carrying tube lines are usually fabricated by means of extrusion and final precision drawing in several steps to the final dimension, and the dominating alloys for this application are AA3003 and AA3103, with higher strength and stiffness compared with the AA3102 alloy. However, the new requirements for longer-life automotive systems have created a demand for aluminum alloys with processing flexibility and mechanical strength similar or better than the AA3003/AA3103 alloys, but with significantly improved corrosion resistance.

BRIEF SUMMARY OF THE INVENTION

[0010] The present invention provides an extrudable, drawable and brazeable aluminum alloy that has improved corrosion resistance and is suitable for use in thin-wall fluid-carrying tube lines. More particularly, the present invention provides an aluminum alloy suitable for use in heat exchanger tubing or extrusions, as well as for use as finstock for heat exchangers and in foil packaging applications subjected to corrosion, such as salt water. The aluminum alloy of this invention also exhibits improved formability (including grain size) during bending and end-forming operations.

[0011] The invention is generally directed to an aluminum-based alloy comprising 0.06-0.35% by weight of iron, 0.05-0.15% by weight of silicon, 0.01-1.0% by weight of manganese, 0.02-0.60% by weight of magnesium, 0.05-0.70% by weight of zinc, optionally one or more of the elements zirconium, titanium, chromium or copper up to a maximum of 1.30% by weight, up to 0.15% by weight of other impurities, each not greater than 0.03% by weight, and the balance aluminum.

[0012] The iron content for the alloy of this invention is 0.10-0.22% by weight, more preferably 0.17-0.22% by weight based on the processing, mechanical and corrosion resistance properties desired for a fluid-carrying tube of an automotive air-conditioning system, in combination with the other alloy constituents at the levels disclosed below. The corrosion resistance is increased due to smaller amounts of iron rich particles, which generally creates sites for pitting corrosion attack. The relatively low iron content, however, has a negative influence on the final grain size (due to less iron-rich particles acting as nucleation sites for recrystallization). To counterbalance the negative effect of a relatively low iron content in the alloy, other elements must be added for grain structure refinement.

[0013] The presence of magnesium results in a refinement of the final grain size (due to storage of more energy for recrystallization during deformation) as well as improvements in the strain hardening capacity of the material. In total this means improved formability during, for instance, bending and endforming of tubes. Magnesium also has a positive influence on the corrosion properties by altering the oxide layer. The content of magnesium content is preferably not higher than 0.3% by weight due to its strong effect in increasing extrudability. Additions above 0.3% by weight are also incompatible with good brazeability. When used in combination with the other constituents at the stated levels, a preferred magnesium content of 0.10-0.30% by weight promotes the processing, mechanical and corrosion resistance properties desired for a fluid-carrying tube of an automotive air-conditioning system.

[0014] The manganese content is believed to counterbalance the increase in extrusion pressure obtained when adding magnesium, and reduce the negative effect of manganese with respect to precipitation of Mn bearing phases during final annealing. According to the invention, a manganese content of 0.30-0.70 weight percent is necessary to obtain desirable processing, mechanical and corrosion resistance properties when used in combination with the other constituents at the stated levels.

[0015] In view of the polluting effect of zinc (ex., even small zinc concentrations can negatively affect the anodizing properties of AA6000 series alloy), the level of this element is preferably kept low to make the alloy more recycleable and save cost in the cast house. Zinc has a strong positive effect on the corrosion resistance at levels up to at least 0.70% by weight, but for the reasons given above the amount of zinc is preferably between 0.10-0.30% by weight, more preferably between 0.19-0.25% by weight.

[0016] For recycleability, acceptance of chromium in the alloy may be desirable. Addition of chromium, however, increases the extrudability and negatively influences negatively on the tube drawability, and therefore chromium is preferably omitted or otherwise maintained at low levels, e.g., below 0.03% by weight. Zirconium and titanium are believed to improve corrosion resistance at levels of between 0.10-0.18% and 0.10-0.16% by weight, respectively, though surprisingly excellent corrosion resistance is exhibited by preferred alloys in which zirconium is omitted and titanium is kept to levels of at most 0.03% by weight. Finally, the copper content of the alloy should be kept as low as possible, preferably below 0.03% by weight, due to the strong negative effect on corrosion resistance and also due to the substantial influence on extrudability even for small additions.

[0017] Based on extensive testing, preferred alloys of this invention consist essentially of, by weight, about 0.17 to about 0.22% iron, about 0.06 to about 0.10% silicon, about 0.30 to about 0.70% manganese, about 0.10 to about 0.30% magnesium, and about 0.19 to about 0.25% zinc, with the balance aluminum and incidental impurities. Alloys within these limited ranges exhibit an excellent balance of properties, including castability, extrudability, drawability, brazeability, corrosion resistance and mechanical properties.

[0018] Other objects and advantages of this invention will be better appreciated from the following detailed description.

DETAILED DESCRIPTION OF THE INVENTION

[0019] In investigations leading up to this invention, the extrudability, drawability, mechanical properties (including formability parameters) and corrosion resistance were investigated for a series of alloy compositions set forth in Table 1 below. The alloys were prepared in a traditional way by DC casting of extrusion ingots. Table 1 indicates the compositions of the alloys in percent by weight, taking into account that each of these alloys may contain up to 0.02% by weight of incidental impurities. Compositions were selected with varying amounts of magnesium, chromium, zinc, zirconium and titanium. Table 1 also shows the compositions of the standard alloys M3003 and M3103, which were used as reference alloys in the investigation. TABLE 1 Chemical composition of alloys (weight %) Alloy Fe Si Mn Mg Cr Zn Cu Zr Ti ES1 0.20 0.08 0.45 0.17 — 0.19 — — — ES2 0.20 0.08 0.34 0.11 — 0.19 — — — AC1 0.24 0.08 0.67 0.29 — — — — — AC2 0.23 0.09 0.70 0.29 0.10 — — — — AC3 0.24 0.08 0.70 0.27 0.22 — — — — AC4 0.21 0.08 0.68 0.28 — 0.25 — — — AC5 0.20 0.08 0.67 0.27 0.07 0.24 — — — AC9 0.25 0.13 0.67 0.05 0.04 0.16 — — 0.16 AC10 0.22 0.10 0.74 0.29 — 0.13 0.001 — — AC13 0.22 0.10 0.71 0.27 0.12 0.22 0.001 — — AC18 0.22 0.10 0.50 0.26 — 0.22 0.001 — — AA3103 0.54 0.11 1.02 — — — 0.03  — 0.01 AA3003 0.59 0.27 1.05 0.01 — — 0.08  — 0.01

[0020] The following description details the techniques used to investigate the properties, followed by a discussion of the obtained results.

[0021] The compositions of the billets were determined by means of electron spectroscopy. For this analysis a Baird Vacuum Instrument was used, and the test standards as supplied by Pechiney, were used. Extrusion billets were homogenized according to standard routines, using a heating rate of about 100° C./hr to a holding temperature of approximately 600° C., followed by air cooling to room temperature.

[0022] Extrusion of the homogenized billets were carried out on a full scale industrial extrusion press using the following conditions: Billet temperature: 455-490° C. Extrusion ration: 63:1 Ram speed: 16.5 mm/sec Die: three hole Extrudate: 28 mm OD tube (extrudate water cooled)

[0023] The extrudability was related to the die pressure and the maximum extrusion pressure (peak pressure). Those parameters were registered by pressure transducers mounted on the press, giving a direct read out of these values.

[0024] The extruded base tubes were finally plug drawn in a total of six draws to a final 9.5 mm OD tube with a 0.4 mm wall. The reduction in each draw was approximately 36%. After the final draw the tubes were soft annealed in a batch furnace at a temperature of about 420° C. Testing of mechanical properties of the annealed tubes was carried out on a Schenk Trebel universal tensile testing machine in accordance with the Euronorm standard. In the testing, the E-module was fixed to about 70000 N/mm² during the entire testing. The speed of the test was constant at about 10 N/mm² per second until YS (yield strength) was reached, while the testing from YS until fracture appeared was about 40% Lo/min, Lo being the initial gauge length.

[0025] Corrosion potential measurements were performed according to a modified version of the ASTM G69 standard test, using a Gamry PC4/300 equipment with a saturated calomel electrode (SCE) as a reference. The tube specimens were degreased in acetone prior to measurements. No filing or abrasion of the tube specimen surface was performed, and the measurements were done without any form of agitation. Corrosion potentials were recorded continuously over a sixty minute period and the values presented represents the average of those recorded during the final thirty minutes of the test.

[0026] Corrosion resistance was investigated using the so-called SWAAT test (Acidified Synthetic Sea Water Testing). The test was performed according to ASTM G85-85 Annex A3, with alternating thirty-minute spray periods and ninety-minute soak periods at about 98% humidity. The electrolyte used was artificial sea water acidified with acetic acid to a pH of about 2.8 to about 3.0 and a composition according to ASTM standard D1141. The temperature in the chamber was kept at about 49° C. The test was run in an Erichsen Salt Spray Chamber (Model 606/1000).

[0027] In order to study the evolution of corrosion behavior, samples from the different alloys were taken out of the chamber every third day. The materials were then rinsed in water and subsequently tested for leaks by immersing tube specimens in water and applying a pressure of about 1 bar. The test as described is in general use within the automotive industry, where an acceptable performance is qualified as being above twenty days exposure.

[0028] Extrusion data for the commercial alloys AA3003 and AA3103 and the experimental alloys AC1-AC5 and AC9 are given in Table 2 below. TABLE 2 Extrusion Data Peak Die Chemical composition wt % Press. Press. Alloy Fe Si Mn Mg Cr Zn Cu Ti (kN) (kN) AC1 0.24 0.08 0.67 0.29 — — — — 2573 1395 AC2 0.23 0.09 0.70 0.29 0.10 — — — 2584 1424 AC3 0.24 0.08 0.70 0.27 0.22 — — — 2597 1464 AC4 0.21 0.08 0.68 0.28 — 0.25 — — 2536 1373 AC5 0.20 0.08 0.67 0.27 0.07 0.24 — — 2559 1415 AC9 0.25 0.13 0.67 0.05 0.04 0.24 — 0.16 2552 1385 3103 0.54 0.11 1.02 — — — 0.03 0.01 2399 1281 3003 0.59 0.27 1.05 0.01 — — 0.08 0.01 2481 1288

[0029] As seen from Table 2 the extrusion pressures obtained for the experimental alloys are approximately 5-6% higher than for alloys AA3103 and M3003. This was regarded as a small difference, and it should be noted that all alloys were run at the same billet temperature and ram speed (no press-parameter optimization done in this test).

[0030] Surface finish after extrusion, especially on the interior of the tube, is particularly important in applications such as fluid-carrying tube lines of interest here, because the tube is to be cold drawn to a smaller diameter and wall thickness. Surface defects may interfere with the drawing process and result in fracture of the tube during drawing. All the investigated alloys in the test matrix showed good internal surface appearance.

[0031] The alloys drew well (same speed and productivity as for standard 3003/3103 alloys), except for the AC10 and AC18 alloys, which periodically broke during the last (third) draw. The characteristics of the alloys after annealing are given in Table 3. Table 3: Characteristics of the alloys after drawing and soft annealing. TABLE 3 Characteristics of the alloys after drawing and soft annealing. Elong. Grain SWAAT Corrosion YS UTS (A10) Size** Life Potential Alloy (MPa) (MPa) (%) n-value* (μm) (days) (mV SCE) ES1 30 88 42.0 0.260 84 36 −760 ES2 37 98 39.0 0.250 65 33 −757 AC1 51 113 36.1 0.244 82 7 −769 AC2 52 115 36.1 0.236 56 15 −755 AC3 53 117 37.1 0.232 66 15 −760 AC4 46 112 36.0 0.250 88 57 −769 AC5 51 113 36.6 0.237 79 41 −782 AC9 42 99 43.0 0.238 92 30 −830 AC10 49 112 37.8 0.238 83 32 −797 AC13 51 121 36.9 0.227 59 49 −819 AC18 63 106 37.2 0.218 59 25 −745 AA3103 48 108 41.2 0.232 141 3 −730 AA3003 48 108 39.8 0.241 70 3 −754

[0032] From the results in Table 3 it can be seen that the mechanical properties, grain size and corrosion resistance are alloy dependent. The mechanical properties of the test alloys are slightly higher compared with the standard alloys. Note also the refinement in grain structure obtained for the test alloys compared with AA3103.

[0033] Of particular interest, the corrosion resistance (in terms of SWAAT life) of all the test alloys is superior compared to the standard alloys AA3003 and AA3103, whose tubes failed after only three days. Note that the SWAAT life, given for each alloy in Table 3, represents the first tube parallel to fail out of a total of 10 parallels mounted in the SWAAT chamber. Analysis of the SWMT data shows that adding Mg while reducing Mn and Fe had the general effect of increasing SWMT life as compared to the AA3003/3103 alloys. However, in comparing the AC4 and AC18 alloys, reducing manganese levels can significantly reduce corrosion resistance for some combinations of constituents.

[0034] Additions of Zn also appeared to be beneficial to corrosion resistance. The effect of zinc on corrosion resistance is particularly evident when comparing the AC4 and AC10 alloys. However, an evaluation of an alloy having essentially the same composition as AC4 but with a zinc level of about 0.33 weight percent was found to break periodically during the third drawing.

[0035] Though additions of chromium may improve corrosion resistance (if comparing the AC2 and AC3 alloys to the M3003/3103 alloys), a comparison of the AC4 and AC5 alloys suggests that adding chromium may decrease corrosion resistance for at least some combinations of constituents.

[0036] Finally, when comparing the AC5 and AC13 alloys, titanium appeared to have little effect on corrosion resistance for the particular ranges of compositions evaluated.

[0037] From the above, it is evident that simply increasing or decreasing a single constituent can have a significant effect on properties, such that an alloy having a desired combination of properties can only be determined through testing, with optimization of all desired properties being impractical. Nonetheless, it can be concluded that the best alloy combinations with respect to corrosion were found for a relatively high Zn content (about 0.13 to about 0.25%), and no Cr or Ti added. Additions of Cr may be desirable in terms of recycleability and, although additions of Cr appear to correspond to a reduction in SWAAT, alloys containing Cr still exhibited good corrosion resistance (AC5), particularly in comparison to the standard AA3003 and 3103 alloys. Alloys ES1, ES2, AC4, AC5, AC9, AC10 and AC13 exhibited a SWAAT life of at least ten times longer than the AA3003 and 3103 alloys, which was concluded to be an unexpectedly good improvement. The AC4 alloy completed fifty-seven days of exposure under SWAAT testing before failure, which was considered to be an exceptionally good improvement.

[0038] The superior corrosion resistances observed for the test alloys was believed to be attributable in part to the mode of corrosion attack being limited to generally a lamellar type. This extends the time required for corrosion to penetrate through a given thickness, thereby providing along life alloy. As can be seen from Table 3 the electrochemical corrosion potentials of the test alloys were generally decreased (more negative) as compared to the standard alloys AA3103/AA3003. In order for the tube material not to behave sacrificially towards a filler metal (for instance when connected to cladded header in a condenser) it is recommended to select clad materials that match the electrochemical potential. This is the usual methodology applied when designing components/systems against corrosion, and this will curb any attack of the tube due to galvanic corrosion.

[0039] Finally, the tested alloys were scored for a number of important characteristics. Scoring was on a scale of 0.00 to 1.00, with a score of 1.00 for a given property generally being accorded to the highest value believed attainable or received by either the AA3103 alloy or one of the “AC” alloys for a given property. More particularly: a score of 1.00 was assigned if the alloy was castable (all were); extrudability was judged in comparison to the highly extrudable AA3103 alloy, which was assigned the score of 1.00; a score of 1.00 was assigned if the alloy could successfully undergo six draws without failure; mechanical properties were based on the ultimate tensile strength of AA3103 (108 MPa); formability was based on the formability of the final drawn and annealed tube, with an n-value of 0.3 assigned the score of 1.00; a corrosion P(10) score of 1.00 was assigned for a SWMT life of 100 days; electrochemical compatibility was related to the use of the alloy in a heat exchanger, where the tubes would physically contact a fin pack—for scoring, the fin material is assumed to be AA3003 having a corrosion potential of −754 mV, and a full score of 1.00 is assigned for a tube potential more positive than the AA3003; brazeability was based on filler metal flow, and a score of 1.00 was assigned alloys free of Mg based on studies that indicate a 4% decrease in filler metal flow for every 0.1 wt. % addition of Mg. A summary of these weighted scores is provided in Table 4 below. TABLE 4 Alloy Cast. Extrud. Draw. Mech. Form. Corr. Elec. Braze. Total ES1 1.00 1.08 1.00 1.19 0.87 0.72 0.99 0.95 7.80 ES2 1.00 1.00 1.00 1.09 0.83 0.66 0.99 0.93 7.50 AC1 1.00 0.93 0.93 0.95 0.81 0.20 0.98 0.88 6.75 AC2 1.00 0.92 1.00 0.94 0.79 0.29 1.00 0.88 6.82 AC3 1.00 0.92 1.00 0.92 0.77 0.27 0.99 0.89 6.76 AC4 1.00 0.94 1.00 0.96 0.83 0.82 0.98 0.89 7.42 AC5 1.00 0.93 1.00 0.95 0.79 0.82 0.96 0.89 7.34 AC9 1.00 0.94 1.00 1.00 0.79 0.57 0.90 0.98 7.18 AC10 1.00 0.93 1.00 0.96 0.79 0.59 0.94 0.88 7.09 AC13 1.00 0.92 1.00 0.88 0.76 0.63 0.91 0.89 6.99 AC18 1.00 0.96 1.00 1.00 0.73 0.55 1.00 0.90 7.14 AA3103 1.00 1.00 1.00 1.00 0.77 0.04 1.00 1.00 6.81

[0040] From the results in Table 4, it can be seen that the ES1, ES2 and AC4 alloys provided the best balance of the properties investigated. Even so, it is readily apparent these alloys, which have a desired combination of properties for thin-wall fluid-carrying tube lines used in a corrosive environment, could only have been distinguished from the remaining alloys (with similar compositions) through the testing of the type described above. Based on the similarities of the ES1, ES2 and AC4 alloys, a suitable composition for an alloy of this invention is believed to consist essentially of, by weight, about 0.17 to about 0.22% iron, about 0.06 to about 0.10% silicon, about 0.30 to about 0.70% manganese, about 0.10 to about 0.30% magnesium, and about 0.19 to about 0.25% zinc, with the balance aluminum. Suitable approximate ranges for alloys based on the preferred ES1, ES2 and AC4 alloys are summarized in Table 5 below. TABLE 5 Alloy Fe Si Mn Mg Zn ES1 0.17-0.22 0.06-0.10 0.40-0.52 0.15-0.20 0.19-0.25 ES2 0.17-0.22 0.06-0.10 0.30-0.40 0.10-0.15 0.19-0.25 AC1 0.18-0.22 0.06-0.10 0.62-0.70 0.25-0.30 0.20-0.25

[0041] While the invention has been described in terms of preferred embodiments, it is apparent that other forms could be adopted by one skilled in the art. Therefore, the scope of the invention is to be limited only by the following claims. 

1. An aluminum alloy consisting essentially of, by weight, about 0.17 to about 0.22% iron, about 0.06 to about 0.10% silicon, about 0.30 to about 0.70% manganese, about 0.10 to about 0.30% magnesium, and about 0.19 to about 0.25% zinc, with the balance essentially aluminum and incidental impurities.
 2. An aluminum alloy according to claim 1, wherein the alloy contains 0.18 to 0.22 weight percent iron.
 3. An aluminum alloy according to claim 1, wherein the alloy contains 0.30 to 0.40 weight percent manganese.
 4. An aluminum alloy according to claim 1, wherein the alloy contains 0.40 to 0.52 weight percent manganese.
 5. An aluminum alloy according to claim 1, wherein the alloy contains 0.62 to 0.70 weight percent manganese.
 6. An aluminum alloy according to claim 1, wherein the alloy contains 0.10 to 0.15 weight percent magnesium.
 7. An aluminum alloy according to claim 1, wherein the alloy contains 0.15 to 0.20 weight percent magnesium.
 8. An aluminum alloy according to claim 1, wherein the alloy contains 0.25 to 0.30 weight percent magnesium.
 9. An aluminum alloy according to claim 1, wherein the alloy contains 0.20 to 0.25 weight percent zinc.
 10. An aluminum alloy according to claim 1, wherein the alloy consists of, by weight, 0.17 to 0.22% iron, 0.06 to 0.10% silicon, 0.30 to 0.40% manganese, 0.10 to 0.15% magnesium, and 0.19 to 0.25% zinc, with the balance essentially aluminum and incidental impurities.
 11. An aluminum alloy according to claim 1, wherein the alloy consists of, by weight, 0.17 to 0.22% iron, 0.06 to 0.10% silicon, 0.40 to 0.52% manganese, 0.15 to 0.20% magnesium, and 0.19 to 0.25% zinc, with the balance essentially aluminum and incidental impurities.
 12. An aluminum alloy according to claim 1, wherein the alloy consists of, by weight, 0.18 to 0.22% iron, 0.06 to 0.10% silicon, 0.62 to 0.70% manganese, 0.25 to 0.30% magnesium, and 0.20 to 0.25% zinc, with the balance essentially aluminum and incidental impurities.
 13. A fluid-carrying tube of an automotive air-conditioning system, the tube being drawn from an aluminum alloy consisting of, by weight, about 0.17 to about 0.22% iron, about 0.06 to about 0.10% silicon, about 0.30 to about 0.70% manganese, about 0.10 to about 0.30% magnesium, and about 0.19 to about 0.25% zinc, with the balance essentially aluminum and incidental impurities.
 14. A fluid-carrying tube according to claim 13, wherein the alloy contains 0.30 to 0.40 weight percent manganese.
 15. A fluid-carrying tube according to claim 13, wherein the alloy contains 0.40 to 0.52 weight percent manganese.
 16. A fluid-carrying tube according to claim 13, wherein the alloy contains 0.62 to 0.70 weight percent manganese.
 17. A fluid-carrying tube according to claim 13, wherein the alloy contains 0.10 to 0.15 weight percent magnesium.
 18. A fluid-carrying tube according to claim 13, wherein the alloy contains 0.15 to 0.20 weight percent magnesium.
 19. A fluid-carrying tube according to claim 13, wherein the alloy contains 0.25 to 0.30 weight percent magnesium.
 20. A fluid-carrying tube according to claim 13, wherein the alloy consists of, by weight, about 0.20% iron, about 0.08% silicon, about 0.34% manganese, about 0.11% magnesium, and about 0.19% zinc, with the balance essentially aluminum and incidental impurities.
 21. A fluid-carrying tube according to claim 13, wherein the alloy consists of, by weight, about 0.20% iron, about 0.08% silicon, about 0.45% manganese, about 0.17% magnesium, and about 0.19% zinc, with the balance essentially aluminum and incidental impurities.
 22. A fluid-carrying tube according to claim 13, wherein the alloy consists of, by weight, about 0.21% iron, about 0.08% silicon, about 0.68% manganese, about 0.28% magnesium, and about 0.25% zinc, with the balance essentially aluminum and incidental impurities. 